Abstract

During the Galileo flybys of Callisto in 1999, a CO_2 atmosphere and an ionosphere were detected. Using the Caltech/Jet Propulsion Laboratory one-dimensional KINETICS model, we have successfully simulated the observed electron density within a factor of 2, while satisfying the observational constraints on carbon and oxygen atoms. We conclude that photoionization of CO_2 alone is insufficient to produce the observed electron density. An atmosphere 20–100 times denser than the CO_2 atmosphere must be introduced, as suggested by Kliore et al. (2002). We show that an O_2-rich atmosphere is highly probable. However, the atomic oxygen produced from O_2 photodissociation is 2 orders of magnitude greater than the upper limit given by Strobel et al. (2002). The introduction of reactive hydrogen chemistry assuming a surface abundance of H_2O of ∼2 × 10^9 cm^(−3) (4 × 10^(−8) mbar) is required to reduce the excess atomic O abundance. The calculated atomic O column density is >5 × 10^(12) cm^(−2), which is about the observed upper limit, suggesting we should be able to detect O in the atmosphere of Callisto.